Aberration-correcting method, laser processing method using said aberration-correcting method, laser irradiation method using said aberration-correction method, aberration-correction device and aberration-correcting program

ABSTRACT

In an aberration-correcting method according to an embodiment of the present invention, in an aberration-correcting method for a laser irradiation device  1  which focuses a laser beam on the inside of a transparent medium  60 , aberration of a laser beam is corrected so that a focal point of the laser beam is positioned within a range of aberration occurring inside the medium. This aberration range is not less than n×d and not more than n×d+Δs from an incidence plane of the medium  60 , provided that the refractive index of the medium  60  is defined as n, a depth from an incidence plane of the medium  60  to the focus of the lens  50  is defined as d, and aberration caused by the medium  60  is defined as Δs.

This is a continuation application of copending application Ser. No.13/061,438, having a §371 date of Apr. 26, 2011, which is a nationalstage filing based on PCT International Application No.PCT/JP2009/064939, filed on Aug. 27, 2009. The copending applicationSer. No. 13/061,438 is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to an aberration-correcting method forcorrecting an aberration of a laser irradiation device, a laserprocessing method using said aberration-correcting method, a laserirradiation method using said aberration-correcting method, anaberration-correcting device, and an aberration-correcting program.

BACKGROUND ART

A laser irradiation device is used in various optical devices such aslaser processing devices and microscopes. Some laser processing devicesusing said laser irradiation device includes a special light modulator(hereinafter, referred to as an SLM). Patent Literatures 1 to 5 listedbelow disclose laser processing devices including an SLM.

The laser processing devices described in Patent Literatures 1 and 2control a laser beam irradiation position in a processing target, andthe laser processing device described in Patent Literature 3 controls alaser beam by using an SLM. The laser processing device described inPatent Literature 4 includes a means for measuring wavefront distortionof a laser beam, and corrects measured wavefront distortion by using anSLM. However, this method needs a means for measuring wavefrontdistortion, and the optical system thereby becomes complicated, and thismethod is not applicable to the case where measurement of wavefrontdistortion is impossible such as laser processing.

Patent Literature 5 describes a problem in that aberration occurs when alaser beam is focused on a transparent medium, and a processing pointbecomes longer in a depth direction, and the laser processing devicedescribed in Patent Literature 5 positively utilizes chromaticaberration caused by medium dispersion, etc., and optical path changeaccording to a wavelength on a diffraction element, and controls aprocessing position by adjusting an intensity of each of light sourcewavelengths.

Patent Literature 6 describes a method for correcting aberration byapplying a phase distribution opposite to known aberration to incidentlight by a wavefront control element such as an SLM. Here, in Non-PatentLiterature 1, spherical aberration caused by inserting a plane parallelplate into an optical system is analytically obtained according toparaxial approximation. Focusing of a laser beam on a transparent mediumis equivalent to insertion of a plane parallel plate into an opticalsystem, so that by handling the results described in Non-PatentLiterature 1 as known aberration in the method of Patent Literature 6,spherical aberration caused by focusing of a laser beam on a transparentmedium can be corrected. However, in this method, the phase range of thephase distribution opposite to the aberration becomes larger over theperformance of the wavefront control element, so that the method is notapplicable when the laser is irradiated deep inside the medium. Further,an accurate laser irradiation position cannot be obtained.

CITATION LIST Patent Literature

-   [Patent Literature 1] Japanese Patent Application Laid-Open    Publication No. 2006-68762-   [Patent Literature 2] Japanese Patent Application Laid-Open    Publication No. 2006-119427-   [Patent Literature 3] Japanese Patent Application Laid-Open    Publication No. 2002-207202-   [Patent Literature 4] Japanese Patent Application Laid-Open    Publication No. 2006-113185-   [Patent Literature 5] Japanese Patent Application Laid-Open    Publication No. 2005-224841-   [Patent Literature 6] WO 2003/036368

Non Patent Literature

-   [Non-Patent Literature 1] Hiroshi Kubota, “Optics”, Iwanami Shoten,    Publishers, 1967, pp. 127-128 and 300-301

SUMMARY OF INVENTION Technical Problem

It has been demanded that laser processing devices perform finerprocessing. For example, in a case where a modified layer such as anoptical waveguide is formed, a focal point as small as possible has beendemanded. However, if the processing position is deep, the focal regionis expanded by aberration, so that it becomes difficult to maintain anexcellent processing state.

Therefore, an object of the present invention is to provide anaberration-correcting method capable of improving the concentration of afocused laser beam even if a laser is irradiated deep inside a medium, alaser processing method using said aberration-correcting method, a laserirradiation method using said aberration-correcting method, anaberration-correcting device, and an aberration-correcting program.

Solution to Problem

The inventors of the present invention repeatedly and earnestlyconducted studies, and as a result, found that as a laser irradiationposition in a medium becomes deep, a PV (peak to valley) value (a PVvalue is a difference between a maximum value and a minimum value ofwavefront aberration, and is equivalent to a phase modulation amount) ofa wavefront for correcting a laser beam increases and exceedsperformance of an element that controls a wavefront such as a spatiallight modulator or the like, so that it becomes impossible tosufficiently correct aberration. A spatial light modulator capable ofcontrolling wavefront is a phase modulation-type spatial light modulatorwhich applies a voltage to an independent pixel, a deformable mirrorwhich deforms a membrane mirror by an actuator, or the like. Generally,a phase modulation range physically applicable by a phasemodulation-type spatial light modulator which applies a voltage to anindependent pixel is approximately 2π to 6π. This range is referred toas a physical phase modulation range. However, by using the phasewrapping technique, the effective phase modulation range can be widenedto several tens of wavelength. The phase modulation range that iseffective and widened by the phase wrapping technique is referred to asan effective phase modulation range. The phase wrapping technique is atechnique for wrapping a phase distribution having a value exceeding thephysical phase modulation range into the physical phase modulation rangeby using the fact that the phase 0 is equal to 2nπ (n is an integer).However, on a wavefront for correcting a laser beam, if a difference inphase modulation amount between adjacent pixels in the spatial lightmodulator exceeds the physical phase modulation range, the phasewrapping technique cannot be applied. Therefore, if the difference inphase modulation amount between adjacent pixels in the spatial lightmodulator exceeds the physical phase modulation range, the wavefront forcorrecting aberration cannot be sufficiently reproduced, and theconcentration of the focused laser beam decreases, and excellentprocessing becomes difficult. In other spatial light modulators such asdeformable mirrors, the physical phase modulation range is larger thanin the phase modulation-type spatial light modulator that applies avoltage to an independent pixel, however, the phase range capable ofbeing modulated is limited, so that when the laser irradiation positionbecomes deep, aberration cannot be sufficiently corrected. In the caseof a deformable mirror, only a spatially continuous phase distributioncan be modulated, and the phase wrapping technique cannot be applied, sothat the physical phase modulation range is equal to the effective phasemodulation range.

Then, the inventors of the present invention found that when aberrationof a laser beam is corrected so that the position in the optical axisdirection of the focal point of the laser beam after being corrected iswithin a range between the position in the optical axis direction of thefocal point of a paraxial ray before being corrected and a focal pointof an outermost ray before being corrected, that is, a range in whichlongitudinal aberration is present inside a medium, the PV value ofwavefront modulation supplied for aberration correction is reduced. As aresult of reduction in the PV value of wavefront modulation foraberration correction, even when the focal position is deep, thedifference in phase modulation amount between adjacent pixels in thespatial light modulator is reduced, and it becomes possible to apply thephase wrapping technique. Hereinafter, the wavefront modulation patternfor aberration correction before phase wrapping is applied will bereferred to as a correction wavefront, and a pattern obtained byapplying phase wrapping to the correction wavefront will be referred toas an aberration correction phase pattern.

The aberration-correcting method of the present invention ischaracterized in that, in an aberration-correcting method for a laserirradiation device which focuses a laser beam on the inside of atransparent medium, aberration of a laser beam is corrected so that afocal point of the laser beam is positioned in an aberration rangegenerated inside the medium. Here, “a focal point of the laser beam ispositioned in an aberration range generated inside the medium” meansthat a focal point is positioned within a range in which longitudinalaberration is present inside the medium when the aberration is notcorrected.

According to this aberration-correcting method, aberration of a laserbeam is corrected so that the focal point of the laser beam ispositioned in an aberration range generated inside the medium, that is,positioned within a range in which longitudinal aberration is presentinside the medium when aberration is not corrected, so that the PV valueof the wavefront can be reduced. As a result, even when a spatial lightmodulator the phase modulation amount of which is limited is used, byreducing the phase modulation amount for aberration correction, the taskon the spatial light modulator is reduced, and precise wavefront controlis possible. As a result, even if the laser is irradiated deep insidethe medium, the concentration of the focused laser beam can beincreased, and an excellent processing state can be maintained.

The above-described laser irradiation device includes a focusing meansfor focusing a laser beam on the inside of a medium, and when therefractive index of the medium is defined as n, a depth from anincidence plane of the medium to the focus of the focusing means on theassumption that the refractive index n of the medium is equal to that ofa focusing means atmospheric medium (hereinafter, referred to as amedium movement distance) is defined as d, and a maximum value oflongitudinal aberration caused by the medium is defined as Δs, theabove-described longitudinal aberration range is approximately not lessthan n×d and not more than n×d+Δs from the incidence plane of themedium, and in the above-described aberration-correcting method,aberration of the laser beam is corrected so that the focal point of thelaser beam is positioned within a range more than n×d and less thann×d+Δs from the incidence plane of the medium.

The above-described laser irradiation device includes a condenser lensfor focusing a laser beam on the inside of the medium, and a spatiallight modulator for correcting aberration of a laser beam, and in theabove-described aberration-correcting method, a phase difference betweena phase modulation amount at an arbitrary pixel on the spatial lightmodulator corresponding to an incidence portion of the condenser lensand a phase modulation amount at a pixel adjacent to said pixel is notmore than a phase range to which the phase wrapping technique isapplicable.

With this configuration, the phase difference between adjacent pixelsbecomes smaller, so that phase wrapping in the spatial light modulatorthe physical phase modulation range of which is limited can be realized,and precise wavefront control is possible.

In the above-described aberration-correcting method, the focal point ofthe laser beam is set so that the phase value of the correctionwavefront has a local maximum point and a local minimum point.

By thus setting the focal point so that the phase value of thecorrection wavefront has a local maximum point and a local minimumpoint, the PV value of the correction wavefront can be reduced.

According to the laser processing method of the present invention, in alaser processing method for a laser processing device including a lightsource for generating a laser beam, a spatial light modulator formodulating the phase of a laser beam from the light source, and acondenser lens for focusing the laser beam from the spatial lightmodulator on a processing position inside a processing target, aprocessing position inside the processing target is set, and a relativemovement distance of the processing target is set so that the processingposition is positioned within a range in which longitudinal aberrationis present inside the processing target when the aberration is notcorrected, a correction wavefront is calculated so that the laser beamis focused on the processing position, and displayed on the spatiallight modulator, a focal position is relatively moved so that thedistance between the processing target and the condenser lens becomesthe relative movement distance, and the laser beam from the light sourceis irradiated onto the processing position in the processing target.

According to this laser processing method, the processing position isset in a range in which longitudinal aberration is present inside theprocessing target when the aberration is not corrected, and aberrationof a laser beam is corrected by the spatial light modulator so that thefocal point of the laser beam is positioned at this processing position,so that the PV value of the wavefront can be reduced. As a result, evenwhen a spatial light modulator the phase modulation amount of which islimited is used, by reducing the phase modulation amount for aberrationcorrection, the task on the spatial light modulator is reduced, andprecise wavefront control is realized. As a result, even if the laser isirradiated deep inside the processing target, the concentration of thefocused laser beam can be increased, and an excellent processing statecan be maintained.

Further, according to the laser processing method of the presentinvention, in a laser processing method for an inner-medium laserfocusing device including a light source for generating a laser beam, aspatial light modulator for modulating the phase of a laser beam fromthe light source, and a condenser lens for focusing the laser beam fromthe spatial light modulator on a predetermined focal position inside amedium, the focal position inside the medium is set, and a relativemovement distance of the medium is set so that the focal position ispositioned within a range in which longitudinal aberration is presentinside the medium when the aberration is not corrected, a correctionwavefront is calculated so that the laser beam is focused on the focalposition, and displayed on the spatial light modulator, the focalposition is relatively moved so that the distance between the medium andthe condenser lens becomes the relative movement distance, and the laserbeam from the light source is irradiated onto the focal position in themedium.

According to this laser irradiation method, the focal position is set ina range in which longitudinal aberration is present inside the mediumwhen the aberration is not corrected, and aberration of the laser beamis corrected by the spatial light modulator so that the focal point ofthe laser beam is positioned at this focal position, so that the PVvalue of the wavefront can be reduced. As a result, even when a spatiallight modulator the phase modulation amount of which is limited is used,by reducing the phase modulation amount for aberration correction, thetask on the spatial light modulator is reduced, and precise wavefrontcontrol is realized. As a result, even if the laser is irradiated deepinside the medium, the concentration of the focused laser beam can beincreased, and an excellent focused state can be maintained.

Another aberration-correcting method of the present invention includes,in an aberration-correcting method for a laser irradiation device whichfocuses a laser beam on the inside of a transparent medium, (a) a firstcorrection wavefront generation step for obtaining a plurality ofcorrection wavefronts which are for correcting aberration of a laserbeam so that a focal point of the laser beam is positioned within arange in which longitudinal aberration is present inside a medium whenthe aberration is not corrected, and correspond to a plurality ofprocessing positions inside the medium, and a plurality of distances(medium movement distances), which are correspond to the plurality ofprocessing positions inside the medium, from a medium surface to thepositions of the focal points when the medium is absent; (b) a firstpolynomial approximation step for obtaining a first high-orderpolynomial by performing high-order polynomial approximation of theplurality of distances from the medium surface to the positions of thefocal points when the medium is absent; (c) a second polynomialapproximation step for obtaining a plurality of second high-orderpolynomials by performing high-order polynomial approximation of theplurality of correction wavefronts; (d) a third polynomial approximationstep for obtaining a plurality of third high-order polynomials includingthe processing positions as parameters by performing high-orderpolynomial approximation of a plurality of coefficient sequencesconsisting of coefficients of the same order terms in the plurality ofsecond high-order polynomials; (e) a storing step for storingcoefficients of a plurality of order terms in the first high-orderpolynomials and coefficients of a plurality of order terms in theplurality of third high-order polynomials; and (f) a second correctionwavefront generation step for obtaining a correction wavefront of anarbitrary processing position by using a second high-order polynomial byobtaining the second high-order polynomial of the arbitrary processingposition equivalent to the plurality of the second high-orderpolynomials by using coefficients of a plurality of order terms in thefirst high-order polynomial, the first high-order polynomial,coefficients of a plurality of order terms in the plurality of thirdhigh-order polynomials, and the plurality of third high-orderpolynomials.

An aberration-correcting device of the present invention includes, in anaberration-correcting device for a laser irradiation device whichfocuses a laser beam on the inside of a transparent medium, (a) a firstcorrection wavefront generation means for obtaining a plurality ofcorrection wavefronts which are for correcting aberration of the laserbeam so that a focal point of the laser beam is positioned within arange in which longitudinal aberration is present inside the medium whenthe aberration is not corrected, and correspond to a plurality ofprocessing positions inside the medium, and a plurality of distances(medium movement distances), which are correspond to the plurality ofprocessing positions inside the medium, from a medium surface topositions of the focal points when the medium is absent; (b) a firstpolynomial approximation means for obtaining a first high-orderpolynomial by performing high-order polynomial approximation of theplurality of distances from the medium surface to the positions of thefocal points when the medium is absent; (c) a second polynomialapproximation means for obtaining a plurality of second high-orderpolynomials by performing high-order polynomial approximation of theplurality of correction wavefronts; (d) a third polynomial approximationmeans for obtaining a plurality of third high-order polynomialsincluding the processing positions as parameters by performinghigh-order polynomial approximation of a plurality of coefficientsequences consisting of coefficients of the same order terms in theplurality of second high-order polynomials; (e) a storing means forstoring coefficients of a plurality of order terms in the firsthigh-order polynomials and coefficients of a plurality of order terms inthe plurality of third high-order polynomials; and (f) a secondcorrection wavefront generation means for obtaining a correctionwavefront of an arbitrary processing position by using a secondhigh-order polynomial by obtaining the second high-order polynomial ofthe arbitrary processing position equivalent to the plurality ofhigh-order polynomials by using coefficients of a plurality of orderterms in the first high-order polynomial, the first high-orderpolynomial, coefficients of a plurality of order terms in the pluralityof third high-order polynomials, and the plurality of third high-orderpolynomials.

An aberration-correcting program of the present invention makes acomputer function as, in an aberration-correcting program for a laserirradiation device which focuses a laser beam on the inside of atransparent medium, (a) a first correction wavefront generation meansfor obtaining a plurality of correction wavefronts which are forcorrecting aberration of the laser beam so that a focal point of thelaser beam is positioned within a range in which longitudinal aberrationis present inside the medium when the aberration is not corrected, andcorrespond to a plurality of processing positions inside the medium, anda plurality of distances (medium movement distances), which arecorrespond to the plurality of processing positions inside the medium,from a medium surface to positions of the focal points when the mediumis absent; (b) a first polynomial approximation means for obtaining afirst high-order polynomial by performing high-order polynomialapproximation of the plurality of distances from the medium surface tothe positions of the focal points when the medium is absent; (c) asecond polynomial approximation means for obtaining a plurality ofsecond high-order polynomials by performing high-order polynomialapproximation of the plurality of correction wavefronts; (d) a thirdpolynomial approximation means for obtaining a plurality of thirdhigh-order polynomials including the processing positions as parametersby performing high-order polynomial approximation of a plurality ofcoefficient sequences consisting of coefficients of the same order termsin the plurality of second high-order polynomials; (e) a storing meansfor storing coefficients of a plurality of order terms in the firsthigh-order polynomials and coefficients of a plurality of order terms inthe plurality of third high-order polynomials; and (f) a secondcorrection wavefront generation means for obtaining a correctionwavefront of an arbitrary processing position by using a secondhigh-order polynomial by obtaining the second high-order polynomial ofthe arbitrary processing position equivalent to the plurality of thesecond high-order polynomials by using coefficients of a plurality oforder terms in the first high-order polynomial, the first high-orderpolynomial, coefficients of a plurality of order terms in the pluralityof third high-order polynomials, and the plurality of third high-orderpolynomials.

According to these other aberration-correcting method andaberration-correcting device and aberration-correcting program of thepresent invention, correction wavefronts for correcting aberration of alaser beam so that a focal point of the laser beam is positioned withina range in which longitudinal aberration is present inside a medium whenthe aberration is not corrected are obtained in advance, and by using anapproximate expression using high-order polynomial approximation of thecorrection wavefronts, a correction wavefront at an arbitrary processingposition is obtained, so that the correction wavefront at the arbitraryprocessing position can correct aberration of the laser beam so that thefocal point of the laser beam is positioned within a range in whichlongitudinal aberration is present inside the medium when the aberrationis not corrected, and the PV value of the wavefront can be reduced. As aresult, even if a spatial light modulator the phase modulation amount ofwhich is limited is used, by reducing the phase modulation amount foraberration correction, the task on the spatial light modulator isreduced, and precise wavefront control becomes possible. As a result,even if a laser is irradiated deep inside a medium, the concentration ofa focused laser beam can be increased, and an excellent processing statecan be maintained.

Here, the shape and size of the aberration differ depending on the focalposition, so that in processing involving a change in processing depth(processing position), the correction wavefront must be re-calculatedfor each change, so that it takes a great deal of calculation time. Forexample, in order to obtain a correction wavefront so that a focal pointof a laser beam is positioned within a range in which longitudinalaberration is present inside a medium when the aberration is notcorrected, proper values must be derived by performing multiplesearching for a plurality of parameters, and this takes a great deal ofcalculation time. As a result, when performing processing while changingthe processing depth, searching during processing causes a lowering inthe processing rate.

However, according to these other aberration-correcting method andaberration-correcting device and aberration-correcting program of thepresent invention, correction wavefronts for a plurality of processingpositions are obtained in advance, and high-order polynomialapproximation of these correction wavefronts is performed, so that aproper correction wavefront can be obtained by only performing anarithmetic operation using the approximate expression. As a result, thetime to be taken for re-calculating a correction wavefront when changingthe processing depth can be shortened, and a lowering in the processingrate can be reduced. Even for an arbitrary processing position differentfrom the processing positions obtained in actuality by theabove-described searching, a proper correction wavefront can beobtained.

According to still another aberration-correcting method of the presentinvention, in an aberration-correcting method for a light irradiationdevice which focuses irradiation light on the inside of a transparentmedium, aberration of irradiation light is corrected so that a focalpoint of the irradiation light is positioned within a range in whichlongitudinal aberration is present inside the medium when the aberrationis not corrected.

According to this aberration-correcting method, aberration ofirradiation light is corrected so that a focal point of the irradiationlight is positioned within a range in which longitudinal aberration ispresent inside the medium when the aberration is not corrected, so thatthe PV value of the wavefront can be reduced. As a result, even if aspatial light modulator the phase modulation amount of which is limitedis used, by reducing the phase modulation amount for aberrationcorrection, the task on the spatial light modulator is reduced andprecise wavefront control is realized. As a result, even if a light isirradiated deep inside the medium, the concentration of the focusedirradiation light can be increased, and an excellent processing statecan be maintained.

Advantageous Effects of Invention

According to the present invention, even if a laser is irradiated deepinside a medium, the concentration of the focused laser beam can beincreased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a configuration of a laser processing device(laser irradiation device, laser focusing device) according to a firstembodiment.

FIG. 2 is a view showing an optical path of a laser beam when a parallelplane is inserted in a focusing optical system.

FIG. 3 is a view showing an optical path of a laser beam when a focalpoint is inside the parallel plane.

FIG. 4 is a diagram showing a phase modulation amount of a correctionwavefront in the focusing optical system of FIG. 3.

FIG. 5 is a view showing an optical path of a laser beam for describingan aberration-correcting method, a laser processing method, and a laserirradiation method according to the first embodiment of the presentinvention.

FIG. 6 is a diagram showing a phase modulation amount of a correctionwavefront in the focusing optical system of FIG. 5.

FIG. 7 is a flowchart showing steps of the aberration-correcting method,the laser processing method, and the laser irradiation method of thefirst embodiment of the present invention.

FIG. 8 show results of measurement of a focused state in a processingtarget when the aberration-correcting method of the first embodiment isused.

FIG. 9 is a view showing a configuration of a laser processing device(laser irradiation device, laser focusing device) according to a secondembodiment and an aberration-correcting device according to anembodiment of the present invention.

FIG. 10 is a diagram showing phase modulation amounts of a plurality ofcorrection wavefronts generated by a first correction wavefrontgeneration means.

FIG. 11 is a diagram showing a plurality of second high-orderpolynomials obtained by a second polynomial approximation means.

FIG. 12 is a graph of a plurality of coefficient sequences consisting ofcoefficients of the same order terms in the plurality of secondhigh-order polynomials shown in FIG. 11.

FIG. 13 is a diagram showing a plurality of third high-order polynomialsobtained by a third polynomial approximation means.

FIG. 14 is a view showing a coefficient data set to be stored in astoring means, which is coefficients of a plurality of order terms inthe plurality of third high-order polynomials shown in FIG. 13, and acoefficient sequence in the first high-order polynomial.

FIG. 15 is a flowchart showing an aberration-correcting method accordingto the second embodiment of the present invention.

FIG. 16 is a view showing a configuration of an aberration-correctingprogram according to an embodiment of the present invention, togetherwith a recording medium.

FIG. 17 is a view showing a hardware configuration of a computer forexecuting a program stored in a recording medium.

FIG. 18 is a perspective view of the computer for executing a programstored in a recording medium.

FIG. 19 is a view showing a configuration of an aberration-correctingdevice and a laser processing device according to an exemplary variationof the present invention.

FIG. 20 is a view showing a configuration of the aberration-correctingdevice and the laser processing device according to the exemplaryvariation of the present invention.

FIG. 21 is a diagram showing a phase modulation amount of a correctionwavefront according to an aberration-correcting method of the secondembodiment.

FIG. 22 are views showing results of measurement of a focused state in aprocessing target by using the correction wavefront shown in FIG. 21.

FIG. 23 shows a result of observation of a cut surface of a processingtarget 60 cut after being subjected to conventional laser processing.

FIG. 24 shows a result of observation of a cut surface of a processingtarget 60 cut after being laser-processed by using theaberration-correcting method of the first embodiment.

FIG. 25 shows a result of observation of a cut surface of a processingtarget 60 cut after being laser-processed by using theaberration-correcting method of the second embodiment.

FIG. 26 is a view showing an example of a light irradiation device usingan aberration-correcting method of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the drawings. Portions identicalor equivalent to each other in the drawings will be designated by thesame reference numeral.

First Embodiment

First, before describing an aberration-correcting method, a laserprocessing method, and a laser irradiation method according to a firstembodiment of the present invention, a laser processing device (laserirradiation device, laser focusing device) using thisaberration-correcting method will be shown.

FIG. 1 is a view showing a configuration of a laser processing device(laser irradiation device, laser focusing device) according to a firstembodiment. A laser processing device 1 shown in FIG. 1 includes a lightsource 10, a lens 20, a mirror 30, a spatial light modulator(hereinafter, referred to as an SLM) 40, and an objective lens (focusingmeans, condenser lens) 50. In FIG. 1, a processing target 60 and ameasurement system 70 for measuring a focused state of a laser beam inthe processing target 60 are shown.

The light source 10 outputs a laser beam. The lens 20 is, for example, acollimating lens, and collimates the laser beam from the light source 10into parallel light. The mirror 30 reflects the laser beam from the lens20 toward the SLM 40, and reflects a laser beam from the SLM 40 towardthe objective lens 50. The SLM 40 is, for example, an LCOS-SLM (LiquidCrystal on Silicon—Spatial Light Modulator), and modulates the phase ofthe laser beam from the mirror 30. The objective lens 50 focuses thelaser beam from the mirror 30 and makes it outgo to the processingtarget 60.

In the present embodiment, a focused state of the laser beam in theprocessing target 60 can be measured with the measurement system 70. Themeasurement system 70 includes a CCD camera and an objective lens.

Next, the concept of the conventional spherical aberration-correctingmethod will be described.

FIG. 2 is a view showing an optical path of a laser beam when a parallelplane is inserted in a focusing optical system. As shown in FIG. 2, whena transparent medium 60 having a parallel plane shape is inserted in afocusing optical system including the condenser lens 50, the focusdeviates by δ from O to O′. This focus deviation value δ changesdepending on the incident height H of incident light that is madeincident on the condenser lens 50. Due to a change in the focal pointposition according to incident light, spherical aberration occurs. Atthis time, the deviation in the optical axis direction from the focalposition of the paraxial ray becomes spherical aberration expressed aslongitudinal aberration (longitudinal spherical aberration), and theaberration becomes maximum at the outermost ray. In this case, themaximum value Δs of the longitudinal aberration is expressed by thefollowing expression (1) by using the expression (14-4) described inSection 14-2 of Non-Patent Literature 1.

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu}{expression}\mspace{14mu} 1} \right\rbrack & \; \\{{\Delta\; s} = {\frac{\left( {n^{\prime 2} - n^{2}} \right)n\; d^{\prime}}{2n^{\prime 3}}\theta_{\max}^{2}}} & (1)\end{matrix}$n: Refractive index of atmospheric medium in focusing optical systemn′: Refractive index of medium 60d′: Thickness of medium 60θ_(max): Incidence angle θ of laser beam to the medium 60, and incidenceangle (=arctan (NA)) of outermost ray of the laser beamThe longitudinal aberration may be expressed as aberration in thelongitudinal direction, longitudinal ray aberration, or longitudinalerror.

FIG. 3 is a view showing an optical path of a laser beam when a focalpoint is inside the parallel plane. As shown in FIG. 3, when the focus Oformed by the condenser lens 50 is inside the transparent medium 60having a parallel plane shape, the focus deviates by δ from O to O′.This focus deviation δ changes depending on the incident height H oflight made incident on the condenser lens 50, so that sphericalaberration occurs. In this case, the maximum value Δs of thelongitudinal aberration is expressed by the following expression (2) bytransforming the expression (14-3) described in Section 14-2 ofNon-Patent Literature 1.

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu}{expression}\mspace{14mu} 2} \right\rbrack & \; \\{{\Delta\; s} = {{- \frac{\left( {n^{\prime 2} - 1} \right)}{2n^{\prime}}}\tan^{2}\theta_{\max}d}} & (2)\end{matrix}$n′: Refractive index of medium 60d: Medium movement distanceθ_(max): Incidence angle θ of laser beam to the medium 60, and incidenceangle of outermost ray of the laser beam

Here, when the focal length of the condenser lens 50 is defined as f,according to the spherical aberration Δs of the expression (2) describedabove, wavefront aberration E(h) is expressed by the followingexpression (3) by using the expression (28-6) of Section 28-1 ofNon-Patent Literature 1.

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu}{expression}\mspace{14mu} 3} \right\rbrack & \; \\{{E(h)} = {\frac{1}{f^{2}}{\int_{0}^{h}{\left( {\Delta\; s} \right)h{\mathbb{d}h}}}}} & (3)\end{matrix}$

According to Patent Literature 6, in order to correct the sphericalaberration Δs of the expression (2) described above, the wavefrontbefore focusing, that is, the wavefront to be made incident on thecondenser lens 50 is made opposite to the wavefront aberration E(h) ofthe expression (3). In the laser processing device 1 of the firstembodiment, a pattern obtained by applying phase wrapping to a wavefrontopposite to the wavefront aberration E(h) of the expression (3) is usedas an aberration correction phase pattern of the SLM 40. In this case,the maximum value Δs of longitudinal aberration is expressed by thedeviation of the focal position from the paraxial ray, so that the focalpoint after correction almost matches the focal position of the paraxialray before correction. However, the aberration is calculated byapproximation, so that an accurate focal position cannot be obtained.

For example, when the focal length of the lens 50 is f=4 mm, therefractive index of the processing target 60 is n′=1.49, the numericalaperture is NA=0.8, the laser wavelength is 660 nm, and the mediummovement distance is d=0.9 mm, the focal depth after correctionapproximately becomes medium movement distance d×refractive index n′,and is at the position of 1.34 mm from the medium surface. Thecorrection wavefront in this case becomes a correction pattern shown inFIG. 4, and the phase modulation amount of the correction wavefrontbecomes not less than 600 radians.

As the processing position O′ by the laser processing device 1 becomesdeeper, the spherical aberration Δs increases, so that the phasemodulation amount of the correction wavefront becomes large, and theresolution of the SLM 40 is insufficient, and it becomes difficult tocorrect the aberration.

Thus, in the correction to return the focal point of a ray with respectto each incident height to the position of the medium movement distanced×refractive index n′ by supplying a phase distribution opposite towavefront aberration to a wavefront control element upon analyticallycalculating the wavefront aberration, that is, by the correction toadjust the focal point to the side closest to the condenser lens 50 sidein a longitudinal aberration range occurring inside the processingtarget 60, it is difficult to correct the aberration.

Therefore, in the aberration-correcting method, the laser processingmethod, and the laser irradiation method according to the firstembodiment of the present invention, aberration of a laser beam iscorrected so that the focal point of the laser beam is positioned withina range of aberration occurring inside the processing target 60, thatis, within a range in which longitudinal aberration is present insidethe processing target 60 when the aberration is not corrected. In otherwords, aberration of a laser beam is corrected so that a focal point ofthe laser beam is positioned within a range between a focal position inthe depth direction of a ray on the optical axis when the aberration isnot corrected and a focal position in the depth direction of anoutermost ray when the aberration is not corrected. Therefore, in theaberration-correcting method, the laser processing method, and the laserirradiation method of the first embodiment, a correction wavefront iscalculated from optical path length differences of the rays.Specifically, instead of supplying a phase distribution opposite toaberration upon calculating the aberration as described in PatentLiterature 6, on the assumption that all rays to be made incident on thecondenser lens 50 are focused on one point, a correction wavefront iscalculated by inverse ray tracing. In this case, by setting the mediummovement distance d to a proper value, the PV value of the correctionwavefront is made smaller, and aberration correction at a deep positionby a spatial light modulator the physical or effective phase modulationrange of which is limited becomes possible. Further, an accurate focaldepth can be determined.

FIG. 5 is a view showing an optical path of a laser beam for describingthe aberration-correcting method, the laser processing method, and thelaser irradiation method according to the first embodiment of thepresent invention. As shown in FIG. 5, when an incidence angle of a rayonto the processing target 60 before wavefront correction is defined asθ, an incidence angle of a ray onto the processing target 60 afterwavefront correction is defined as θ₁, and the refractive angle isdefined as θ₂, the optical axis heights h₁, h₂, and h are expressed bythe following expressions (4), (5), and (6). In FIG. 5, by performingwavefront correction, the optical path of the laser beam to theprocessing target 60 is different from the optical path before thewavefront correction.[Mathematical expression 4]h ₁=(f cos θ−d)tan θ₁  (4)[Mathematical expression 5]h ₂=(d+Δ)tan θ₂  (5)[Mathematical expression 6]h=f sin θ  (6)

Here, θ₁ and θ₂ are uniquely related to each other according to Snell'slaw, and when θ₁ is given, θ₂ can be obtained, and on the contrary, whenθ₂ is given, θ₁ can be obtained. Further, the incidence angles θ, θ₁,and θ₂ are uniquely related to each other by h=h₁+h₂ and the expressions(4) to (6). When a specific θ₁ or θ₂ is given, by substituting theexpressions (4) and (5) into h=h₁+h₂ and solving the expression (6), θcan be easily determined. However, when a specific θ is given, it isdifficult to analytically obtain θ₁ and θ₂. In order to obtain θ₁ and θ₂with respect to a specific θ, searching is performed. For example,searching is performed by gradually changing the value of θ₁ or θ₂,obtaining θ for each change, and changing θ₁ or θ₂ until θ₁ or θ₂ isobtained with which a desired θ is obtained.

On the other hand, the optical path difference OPD (Optical PathDifference) caused by the processing target 60 is expressed by thefollowing expression (7).

$\begin{matrix}\left\lbrack {{Mathematical}\mspace{14mu}{expression}\mspace{14mu} 7} \right\rbrack & \; \\{{OPD} = {\frac{\left( {{f \times \cos\;\theta} - d} \right)}{\cos\;\theta_{1}} + \frac{n\left( {d + \Delta} \right)}{\cos\;\theta_{2}} - f - {\left( {n - 1} \right) \times d} - \Delta}} & (7)\end{matrix}$“−f−(n−1)×d−Δ” in the expression (7) is a constant term, and is addedfor preventing the OPD value from becoming excessive large.

By setting the value Δ of focus deviation after the wavefront correctionto a proper value so as to reduce a PV value of a correction wavefrontobtained from this expression (7), a phase modulation amount forcorrecting spherical aberration is reduced. Here, a proper value Δ offocus deviation is obtained by, for example, the above-describedsearching. That is, the value Δ of focus deviation is set to an initialvalue of n×d−d, and is gradually changed, and the OPD (θ) is obtainedfor each change, and Δ is gradually changed until the OPD (θ) in therange of θ_(max)≧θ≧−θ_(max) becomes a desired form. d+Δ is a desiredfocal depth, and is a fixed value, so that Δ and d are changed so thatd+Δ is fixed during searching. The focal depth of d+Δ is a fixed value,so that when Δ is determined, the medium movement distance d is alsodetermined by subtracting Δ from the focal depth. Hereinafter, the focaldepth (processing position) of d+Δ which is a fixed value is referred toas D.

In detail, the focal point shift amount Δ and the movement distance dare determined so that a phase difference between a phase modulationamount at an arbitrary pixel on the SLM 40 corresponding to theincidence portion of the condenser lens 50 and a phase modulation amountat a pixel adjacent to said pixel becomes not more than the physicalphase modulation amount. A laser beam shift amount Δ after thecorrection satisfies 0<Δ<Δs.

Accordingly, the phase difference between adjacent pixels becomessmaller, so that the task on the SLM 40 the physical phase modulationamount of which is limited can be reduced.

Accordingly, when the refractive index of the processing target 60 isdefined as n, the medium movement distance is defined as d, and themaximum value of longitudinal aberration caused by the processing target60 is defined as Δs, the focal point of the laser beam is at a positionnot less than n×d and not more than n×d+Δs from the incidence plane ofthe medium, that is, positioned within a range of longitudinalaberration not less than n×d and not more than n×d+Δs.

In the description above, the focal point shift amount Δ is determinedso that the phase difference between a phase modulation amount at anarbitrary pixel on the SLM 40 corresponding to the incidence portion ofthe condenser lens 50 and a phase modulation amount at a pixel adjacentto said pixel becomes not more than a physical phase modulation amount,however, this searching condition is ambiguous, and a plurality of Δsmay become solutions. In order to make easier the determination of theend of searching, the focal point shift amount Δ may be determined basedon a greater detailed searching condition. For example, Δ may bedetermined so that the PV value of the OPD (θ) in the range ofθ_(max)≧θ≧−θ_(max) becomes minimum. Alternatively, the absolute value ofa differential value of the OPD (θ) in the range of θ_(max)≧θ≧−θ_(max)becomes minimum. These two conditions as examples are included in orsubstantially equal to the first condition that the phase differencebetween a phase modulation amount at an arbitrary pixel on the SLM 40and a phase modulation amount at a pixel adjacent to said pixel becomesnot more than the physical phase modulation amount. As the searchingcondition, various other conditions are possible such as “RMS (Root MeanSquare) of OPD (θ) becomes minimum” and “medium movement distance d isexpressed by a specific function using the focal depth D as a variable,”etc.

For example, when the focal length of the condenser lens 50 is f=4 mm,the refractive index of the processing target 60 is n=1.49, the laserwavelength is 660 nm, the numerical aperture is NA=0.8, and the mediummovement distance is d=0.81 mm, Δ as a result of searching by which thePV value of the OPD (θ) becomes minimum is 0.53 mm, a correctionwavefront becomes the correction pattern shown in FIG. 6, and the phasemodulation amount of the correction wavefront is reduced toapproximately 70 radians. The focal depth in this case is D=1.34 mm, andtherefore, although the same focal depth as in the case using theconventional method is realized, the phase modulation amount becomessmaller, so that aberration can be sufficiently corrected.

Referring to FIG. 6, the phase value of this correction wavefront has alocal maximum point at the position of 0 mm, that is, at the opticalaxis position. Further, the phase value has local minimum points at thepositions of 2 mm and −2 mm. Thus, by setting the focal point so thatthe phase value of the correction wavefront has a local maximum pointand a local minimum point, the PV value of the correction wavefront canbe reduced.

FIG. 7 is a flowchart showing steps of the aberration-correcting method,the laser processing method, and the laser irradiation method of thefirst embodiment. First, a focal point is set on the surface of theprocessing target 60, and this position is defined as a processingorigin (Step S01). Next, a processing position (depth) inside theprocessing target 60 is set (Step S02).

Next, based on the above-described searching condition, the movementdistance d of the processing target 60 and the focal point shift amountΔ are set. Accordingly, the movement amount d of the processing target60 and the focal point shift amount Δ are set so that the processingposition is positioned within a range in which longitudinal aberrationis present inside the processing target 60 when the aberration is notcorrected (a range not less than n×d and not more than n×d+Δs from theincidence plane of the processing target 60) (Step S03). The movementdistance d and the shift amount Δ are set so as to become not more thanthe maximum modulation amount of the spatial light modulator (physicalphase modulation range of the spatial light modulator).

Next, a correction wavefront is calculated so that a laser beam isfocused on the processing position set at Steps S02 and S03, anddisplayed on the SLM 40 (Step S04). Next, the processing target 60 ismoved by the movement distance d (Step S05). Next, a laser beam isirradiated and processing is started. Then, the laser beam is focused onthe set processing position by the correction wavefront of the SLM 40(Step S06).

Next, at the end of processing, laser beam irradiation is stopped (StepS07). When there is another processing position, the process is returnedto Step S02, and when there is no other processing position, processingof this processing target 60 is ended (Step S08).

At Step S05, the relative position of the focusing optical systemincluding the SLM 40 and the condenser lens 50 and the processing target60 is changed, so that the condenser lens 50 may be moved instead ofmoving the processing target 60, or both of these may be moved. In thecase where the condenser lens 50 is moved, when the entrance pupil ofthe condenser lens 50 and the SLM 40 are in an image formingrelationship, the condenser lens must be moved as a unit of the focusingoptical system, that is, the condenser lens must be moved together withthe SLM 40.

At Step S01, by focusing the processing laser beam on the surface of theprocessing target 60, a processing origin is determined, however, therelative position of the condenser lens and the processing target may bedetermined by using another means, for example, an auto-focusing deviceor the like.

In the example described above, the wavefront form is controlled byusing a phase modulation-type spatial light modulator which applies avoltage to an independent pixel, however, another spatial lightmodulator such as a deformable mirror may also be used. When a spatiallight modulator such as a deformable mirror the phase modulation rangeof which is not limited to a small range such as 2π is used, acorrection wavefront can be expressed as it is, so that phase wrappingcan be omitted.

According to the aberration-correcting method, the laser processingmethod, and the laser irradiation method according to the firstembodiment, aberration of a laser beam is corrected so that the focalpoint of the laser beam is positioned within a range of aberrationoccurring inside the medium 60, that is, positioned within a range oflongitudinal aberration inside the medium 60 when the aberration is notcorrected, so that the PV value of the wavefront can be reduced. As aresult, even when an SLM 40 the phase modulation amount of which islimited is used, by reducing the PV value of the correction wavefront,the task on the SLM 40 is reduced, and precise wavefront control becomespossible. As a result, even if a laser is irradiated deep inside themedium 60, the concentration of the focused laser beam can be increased,so that an excellent processing state can be maintained.

Thus, the focal position inside the medium (for example, the processingtarget, etc.) 60 on which a laser beam is focused is moved to a positionat which reduction in the PV value of the correction wavefront becomespossible, so that precise wavefront control can be performed whilereducing the task on the SLM 40 by a simple method.

Further, according to the aberration-correcting method, the laserprocessing method, and the laser irradiation method of the firstembodiment, the phase difference between a phase modulation amount at anarbitrary pixel on the SLM 40 corresponding to the incidence portion ofthe condenser lens 50 and a phase modulation amount at a pixel adjacentto said pixel becomes not more than a phase range in which the phasewrapping technique is applicable. Therefore, the task on the SLM 40 thephysical phase modulation range of which is limited is reduced, andprecise wavefront control is possible.

An accurate focal position cannot be obtained by calculating aberrationby approximation as in the case of Patent Literature 6, however, in thepresent invention, an accurate focal position can be obtained.

FIG. 8 show results of measurement of a focused state in the processingtarget 60. FIG. 8(a) is a result of measurement of a focused statebefore correction, and FIG. 8(b) is a result of measurement of a focusedstate after correction. As shown in FIG. 8, although the same focalposition (1.34 mm) as in the case using the conventional method isrealized, the PV value of the correction wavefront is small, so thataberration is sufficiently corrected.

FIG. 23 and FIG. 24 show results of observation of a cut surface of theprocessing target 60 cut after laser processing. In laser processing inFIG. 23 and FIG. 24, three modified layers 60 a, 60 b, and 60 c wereformed by irradiating a laser beam from the direction Z and scanning thelaser beam in the direction Y on the processing target 60. FIG. 23 showsa cut surface when the aberration-correcting method of the firstembodiment of the present invention is not used in laser processing,that is, a cut surface after laser processing using a laser beam whoseaberration is not sufficiently corrected as shown in FIG. 8(a). On theother hand, FIG. 24 shows a cut surface when the aberration-correctingmethod of the first embodiment of the present invention is used in laserprocessing, that is, a cut surface after laser processing using a laserbeam whose aberration has been sufficiently corrected as shown in FIG.8(b). Referring to FIG. 23, aberration correction in laser processing isinsufficient, so that the modified layers 60 a, 60 b, and 60 c are notuniform, and laser processing is insufficient. On the other hand,referring to FIG. 24, aberration correction in laser processing issufficiently performed, so that modified layers 60 a, 60 b, and 60 c areuniform, and laser processing is sufficiently performed.

The aberration-correcting method, the laser processing method, and thelaser irradiation method of the first embodiment are preferablyapplicable to internal processing of a substance having a wavelengthregion with high transmittance, such as glass, silicon, and sapphiresubstrates. In particular, when forming a modified layer, such as anoptical waveguide, or amorphous or crack generating, even if theprocessing position is deep, aberration can be corrected by the spatiallight modulator, and the focal point can be made smaller, and anexcellent processing state can be maintained.

Second Embodiment

In the aberration-correcting method, the laser processing method, andthe laser irradiation method of the first embodiment described above,when the processing position O′ shown in FIG. 5 changes, the sphericalaberration Δs changes, so that each time the processing position O′changes, the correction wavefront must be re-calculated according to theexpression (7), so that the calculation time becomes long.

In detail, as described above, it is difficult to directly obtain θ₁,θ₂, and Δ in the expression (7), and these are obtained by searching inwhich the value of Δ is gradually changed and the correction wavefrontis repeatedly calculated. Further, in each searching by changing Δ, thevalue of θ or the values of θ₁ and θ₂ in the expression (7) must beobtained by the above-described searching method. Specifically, doublesearching is necessary, and it may take a great deal of calculationtime.

As a result, when processing is performed while changing the processingdepth, searching during processing may cause a lowering in theprocessing rate.

Therefore, the inventors of the present invention found anaberration-correcting method which shortens the time by obtaining inadvance aberration correction wavefronts the phase modulation amounts ofwhich are small by using the aberration-correcting method of the firstembodiment, that is, searching described above, and calculating acorrection wavefront at an arbitrary processing position by using anapproximate expression of polynomial approximation of the obtainedaberration correction wavefronts. Hereinafter, an aberration-correctingmethod according to a second embodiment of the present invention whichshortens the time will be described.

First, before describing the aberration-correcting method according tothe second embodiment of the present invention, a laser processingdevice (laser irradiation device, laser focusing device) that uses thisaberration-correcting method will be described.

FIG. 9 is a view showing a configuration of a laser processing device(laser irradiation device, laser focusing device) according to thesecond embodiment. The laser processing device 1A shown in FIG. 9 isdifferent from the laser processing device 1 in a configuration furtherincluding a control unit 80 and an aberration-correcting device 90according to an embodiment of the present invention in the laserprocessing device 1 of the first embodiment. Other components of thelaser processing device 1A are the same as in the laser processingdevice 1.

The control unit 80 receives correction wavefront information from theaberration-correcting device 90 and controls the phase modulation amountof the SLM 40.

The aberration-correcting device 90 is, for example, a computer, andfunctions as a first correction wavefront generation unit 91, a firstpolynomial approximation unit 92, a second polynomial approximation unit93, a third polynomial approximation unit 94, a storing unit 95, and asecond correction wavefront generation unit 96 by executing anaberration-correcting program that will be described later.

The first correction wavefront generation unit 91 receives a numericalaperture NA and a focal length f determined according to the objectivelens 50, and a refractive index n determined according to a medium ofthe processing target 60. The first correction wavefront generation unit91 receives a range of processing depth estimated in advance inprocessing involving a change in processing depth, and a plurality ofprocessing positions (focal depths) D₁, D₂, . . . D_(p) . . . D_(P)around this range. The number and interval of these focal depths are setso that the polynomial approximation described later can be performedwith sufficient accuracy. The first correction wavefront generation unit91 obtains a plurality of correction wavefronts and a plurality ofmedium movement distances d₁, d₂, . . . d_(p), . . . d_(P) correspondingto the plurality of processing positions D₁, D₂, . . . D_(p) . . . D_(P)by searching using the expressions (7) and (4) to (6). Specifically, thefirst correction wavefront generation unit 91 obtains a plurality ofcorrection wavefronts and a plurality of medium movement distancescorresponding to the plurality of processing positions D₁, D₂, . . .D_(p) . . . D_(P) so that a focal point of a laser beam is positionedwithin a range in which longitudinal aberration is present inside themedium when the aberration is not corrected, and the phase modulationamount of the correction wavefront becomes minimum. As a searchingcondition, a condition by which a correction wavefront and a mediummovement distance are uniquely determined for one processing position,for example, a condition by which “the PV value of the correctionwavefront becomes minimum” is applied. Phase modulation amounts Φ_(1x),Φ_(2x), . . . Φ_(px), . . . Φ_(Px) of a plurality of correctionwavefronts obtained in this manner, plotted with respect to a radialposition x, are shown in FIG. 10.

By the above-described operation, medium movement distances d₁, d₂, . .. d_(p) . . . d_(P) and distances Δ₁, Δ₂, . . . Δ_(p), . . . Δ_(P) fromthe point O to the processing positions D₁, D₂, . . . D_(p) . . . D_(P),and correction wavefronts Φ_(1x), Φ_(2x), . . . Φ_(px), . . . Φ_(Px)corresponding to the plurality of processing positions D₁, D₂, . . .D_(p) . . . D_(P) are obtained.

The first polynomial approximation unit 92 approximates a data set ofmedium movement distances d₁, d₂, . . . d_(p), . . . d_(P) by an M-orderexponential polynomial using a desired focal depth as a variable toobtain one first high-order polynomial (the following expression (8)).[Mathematical expression 8]d=c ₁ *D+c ₂ *D ² . . . +c _(m) *D ^(m) . . . +c _(M) *D ^(M)  (8)Here, D is a desired focal depth, and D=d+Δ.

The second polynomial approximation unit 93 approximates the phasemodulation amounts Φ_(1x), Φ_(2x), . . . Φ_(px), . . . Φ_(Px) of theplurality of correction wavefronts shown in FIG. 10 by a Q-orderexponential polynomial using a radial position x as a variable to obtaina plurality of second high-order polynomials as shown in FIG. 11. Aplurality of coefficient sequences a_(1p), a_(2p), . . . a_(qp), . . .a_(Qp) consisting of coefficients of the same order terms in thesesecond high-order polynomials, that is, the coefficient sequence a_(1p)of the first-order terms to the coefficient sequence a_(Qp) of theQ-order terms are graphed with respect to the medium movement distanced_(p) and shown in FIG. 12.

The third polynomial approximation unit 94 approximates the coefficientsequence a_(1p) of the first-order terms to the coefficient sequencea_(Qp) of the Q-order terms of the plurality of second high-orderpolynomials shown in FIG. 12 by a K-order exponential polynomial using amovement distance d as a variable to obtain a plurality of thirdhigh-order polynomials as shown in FIG. 13.

The storing unit 95 stores the coefficients of the plurality of orderterms in the plurality of third high-order polynomials shown in FIG. 13,that is, coefficients b_(1I) to b_(1Q), b₂₁ to b_(2Q), . . . b_(k1) tob_(KQ), . . . b_(K1) to b_(KQ) of the plurality of first-order toQ-order terms and the coefficient sequence c₁, c₂, . . . c_(q), . . .c_(Q) in the first high-order polynomial as a coefficient data set asshown in FIG. 14.

By using the coefficient data set stored by the above-describedoperations, a correction wavefront with respect to a focal depth at anarbitrary position can be generated. Next, a method for generating thecorrection wavefront will be described.

The second correction wavefront generation unit 96 obtains a mediummovement distance d with respect to an arbitrary focal depth D by usingthe coefficients c₁ to c_(Q) in the coefficient data set and the firstpolynomial, and further obtains the first-order tem coefficient A₁ toQ-order term coefficient A_(Q) of the second high-order polynomial withrespect to the arbitrary focal depth D, that is, obtains the secondhigh-order polynomial (the following expression (9)) of the arbitraryprocessing position D equivalent to the plurality of second high-orderpolynomials shown in FIG. 11 by using the coefficients b₁₁ to b_(1Q),b₂₁ to b_(2Q), . . . b_(k1) to b_(kQ), . . . b_(K1) to b_(KQ) in thecoefficient data set and the plurality of third high-order polynomialsshown in FIG. 13.[Mathematical expression 9]Φ′_(px) =A ₁ *x+A ₂ *x ² . . . +A _(q) *x ^(q) . . . +A _(Q) *x^(Q)  (9)The second correction wavefront generation unit 96 obtains a correctionwavefront at an arbitrary processing position d_(n)+Δ by using thesecond high-order polynomial of the expression (9) shown above.

In the description given above, polynomials consisting of first-order tospecific-order exponential terms are used as the first to thirdpolynomials, however, polynomials having other constitutions may also beused. For example, a 0-order exponential term may be added to the firstto third polynomials. Alternatively, a polynomial consisting ofeven-order exponential terms may be used as the second polynomial.Further, polynomials including, for example, a Zernike polynomial, aGauss function, and a Lorentz function, etc., instead of exponentialfunctions, may also be used. In the second and third polynomials, themedium movement distance d is used as a variable, however, a focal depth(processing position) D or a focal point shift amount Δ may also be usedas a variable. When the searching condition is “the medium movementdistance d is expressed by a specific function using the focal depth Das a variable,” the function may be used instead of the expression (8)and the first polynomial approximation step may be omitted.

Next, operations of the aberration-correcting device 90 of the presentembodiment will be described, and the aberration-correcting methodaccording to the second embodiment of the present invention will bedescribed. FIG. 15 is a flowchart showing an aberration-correctingmethod according to the second embodiment of the present invention.

First, when a numerical aperture NA and a focal length f determinedaccording to the objective lens 50 and a refractive index n determinedaccording to the processing target are input, and a plurality ofprocessing positions (focal depths) D₁, D₂, . . . D_(p), . . . D_(P) inand around a range of processing depth range estimated in advance inprocessing involving a change in processing depth are input, by thefirst correction wavefront generation unit 91, according to theaberration-correcting method of the first embodiment, a plurality ofcorrection wavefronts Φ_(1x), Φ_(2x), . . . Φ_(px), . . . Φ_(Px) and aplurality of medium movement distances d₁, d₂, . . . d_(p), . . . d_(P)corresponding to the plurality of focal depths D₁, D₂, . . . D_(p) . . .D_(P) are obtained by searching using the expressions (7) and (4) to (6)described above. Specifically, a plurality of correction wavefrontsΦ_(1x), Φ_(2x), . . . Φ_(px), . . . Φ_(Px) corresponding to theplurality of focal depths D₁, D₂, . . . D_(p) . . . D_(P) are obtainedso that a focal point of a laser beam is positioned within a range inwhich longitudinal aberration is present inside the medium when theaberration is not corrected, and the PV values of the correctionwavefronts become minimum (S11: first correction wavefront generationstep).

Next, the first polynomial approximation unit 92 performs exponentialpolynomial approximation of the plurality of medium movement distancesd₁, d₂, . . . d_(p), . . . d_(P), and as shown in the expression (8),one first high-order polynomial is obtained (S12: first polynomialapproximation step).

Next, the second polynomial approximation unit 93 performs exponentialpolynomial approximation of phase modulation amounts Φ_(1x), Φ_(2x), . .. Φ_(px), . . . Φ_(Px) of the plurality of correction wavefronts shownin FIG. 10, and accordingly, a plurality of second high-orderpolynomials are obtained as shown in FIG. 11. A plurality of coefficientsequences a_(1p), a_(2p), . . . a_(qp), . . . a_(Qp) consisting ofcoefficients of the same order terms in these second high-orderpolynomials, that is, the coefficient sequence a_(1p) of the first-orderterm to the coefficient sequence a_(Qp) of the Q-order term are obtained(S13: second polynomial approximation step).

Next, the third polynomial approximation unit 94 performs exponentialpolynomial approximation of the coefficient sequence a_(1p) of thefirst-order terms to the coefficient sequence a_(Qp) of the Q-orderterms in the plurality of second high-order polynomials shown in FIG.12, and accordingly, as shown in FIG. 13, a plurality of thirdhigh-order polynomials using the movement distance d as a variable areobtained (S14: third polynomial approximation step). Coefficients of aplurality of order terms in these third high-order polynomials, that is,the coefficients b₁₁ to b_(1Q), b₂₁ to b_(2Q), . . . b_(k1) to b_(kQ), .. . b_(K1) to b_(KQ) of the first-order to Q-order terms and thecoefficients c₁, c₂, . . . c_(q), . . . c_(M) of the first to M-th termsof the first high-order polynomial are stored as a coefficient data setin the storing unit 95 as shown in FIG. 14 (S15: storing step).

In order to calculate a correction wavefront from the stored coefficientdata set, in the second correction wavefront generation unit 96, after amedium movement distance with respect to a desired focal depth D andcoefficients of the second polynomial are obtained, a correctionwavefront is calculated. First, by using the coefficients c₁, c₂, . . .c_(q), . . . c_(M) in the coefficient data set and the first polynomial,a medium movement distance d with respect to an arbitrary focal depth Dis obtained. Next, by using the medium movement distance d, thecoefficients b₁₁ to b_(1Q), b₂₁ to b_(2Q), . . . b_(k1) to b_(kQ), . . .to b_(KQ) in the coefficient data set, and the plurality of thirdhigh-order polynomials shown in FIG. 13, the coefficient a_(1p) of thefirst-order term to the coefficient a_(Qp) of the Q-order term of thesecond high-order polynomial of the arbitrary focal depth D areobtained. Specifically, a second high-order polynomial of the arbitraryfocal depth D equivalent to the plurality of second high-orderpolynomials shown in FIG. 11 is obtained in the form of the expression(9). Thereafter, by using this second high-order polynomial in the formof the expression (9), a correction wavefront at the arbitrary focaldepth D is obtained (S16: correction wavefront generation step).

In order to change the processing depth, a correction wavefrontcorresponding to the changed depth is generated by performing Step S16.

In searching at the first correction wavefront generation step S11described above, the condition that “the PV value of the correctionwavefront becomes minimum” is used, however, other conditions can alsobe used. However, the condition to be used must be set so that acorrection wavefront and a medium movement distance are uniquelydetermined for one processing position, and the phase difference betweenadjacent pixels becomes not more than the physical phase modulationamount.

Next, an aberration-correcting program for operating a computer as theaberration-correcting device 90 will be described. FIG. 16 is a viewshowing a configuration of an aberration-correcting program according toan embodiment of the present invention together with a recording medium.

As shown in FIG. 16, the aberration-correcting program P100 is stored ina recording medium 100 and provided. As the recording medium 100, arecording medium such as a floppy disk, a CD-ROM, a DVD, a ROM or thelike, or a semiconductor memory or the like is used by way of example.

FIG. 17 is a view showing a hardware configuration of a computer forexecuting a program recorded on a recording medium, and FIG. 18 is aperspective view of the computer for executing a program stored in arecording medium.

As shown in FIG. 17, the computer 200 includes a reading device 202 suchas a floppy disk drive unit, a CD-ROM drive unit, and a DVD drive unit,etc., a working memory (RAM) 204 in which an operating system isresident, a memory 206 for storing a program stored in the recordingmedium 100, a display device 208 such as a display, a mouse 210 and akeyboard 212 as input devices, a communication device 214 fortransmitting and receiving data, etc., and a CPU 216 which controlsexecution of programs. When the recording medium 100 is inserted in thereading device 202, the computer 202 becomes accessible to theaberration-correcting program P100 stored in the recording medium 100from the recording device 202, and can operate as anaberration-correcting device 90 according to the aberration-correctingprogram P100.

As shown in FIG. 18, the aberration-correcting program P100 may beprovided as a computer data signal 220 superimposed on a carrier wavevia a network. In this case, the computer 200 stores theaberration-correcting program P100 received by the communication device214 in the memory 206, and can execute the aberration-correcting programP100.

As shown in FIG. 16, the aberration-correcting program P100 includes afirst correction wavefront generation module P10, a first polynomialapproximation module P20, a second polynomial approximation module P30,a second polynomial approximation module P40, a storing module P50, anda second correction wavefront generation module P60.

Functions that the first correction wavefront generation module P10, thefirst polynomial approximation module P20, the second polynomialapproximation module P30, the second polynomial approximation moduleP40, the storing module P50, and the second correction wavefrontgeneration module P60 realize the computer are the same as those ofcorresponding elements of the above-described first correction wavefrontgeneration unit 91, the first polynomial approximation unit 92, thesecond polynomial approximation unit 93, the second polynomialapproximation unit 94, the storing unit 95, and the second correctionwavefront generation unit 96.

The present embodiment shows an embodiment in which a computer thatfunctions as the aberration-correcting device 90 is provided integrallyinside the laser processing device, however, it is also possible thatthe computer that functions as the aberration-correcting device 90 isprovided separately outside the laser processing device 1A, andcorrection wavefront information is exchanged between the computer andthe laser processing device (FIG. 19).

It is also possible that computers 90A and 90B are provided inside andoutside the laser processing device 1A, and these two computers 90A and90B realize the aberration-correcting device 90. For example, it ispossible that the first correction wavefront generation unit 91, thefirst polynomial approximation unit 92, the second polynomialapproximation unit 93, the third polynomial approximation unit 94, andthe storing unit 95 as parts of the aberration-correcting device 90 arerealized by the external computer 90A, and the remaining storing unit 95and second correction wavefront generation unit 96 are realized by theinternal computer 90B. According to this, between the external computer90A and the internal computer 90B, that is, the laser processing device1A, the coefficient data set is exchanged via a recording medium or acommunication path, and the contents in the storing unit 95A of theexternal computer 90A are copied into the storing unit 95B of theinternal computer 90B (FIG. 20).

Thus, the aberration-correcting method of the second embodiment and theaberration-correcting device 90 and the aberration-correcting program ofthe present embodiment also have the same advantages as those of theaberration-correcting method of the first embodiment. Specifically, evenin the aberration-correcting method of the second embodiment and theaberration-correcting device 90 and the aberration-correcting program ofthe present embodiment, as described above, correction wavefronts forcorrecting aberration of a laser beam are obtained in advance so that afocal point of the laser beam is positioned within a range oflongitudinal aberration inside the medium when the aberration is notcorrected is obtained, and by using an approximate expression accordingto high-order polynomial approximation of the correction wavefronts, acorrection wavefront at an arbitrary processing position is obtained, sothat the correction wavefront of the arbitrary processing position cancorrect aberration of the laser beam so that the focal point of thelaser beam is positioned within the range of longitudinal aberrationinside the medium when the aberration is not corrected, and the PV valueof the wavefront can be reduced. As a result, even if a spatial lightmodulator the phase modulation amount of which is limited is used, byreducing the phase modulation amount for aberration correction, the taskon the spatial light modulator is reduced, and precise wavefront controlbecomes possible. As a result, even if a laser is irradiated deep insidea medium, the concentration of a focused laser beam can be increased,and an excellent processing state can be maintained.

For example, when the numerical aperture of the lens 50 is NA=0.55, thefocal length is f=3.6 mm, the refractive index of the processing target60 is n=1.49, the medium movement distance is d=0.637 mm, and the focusdeviation is Δ=0.361 mm, that is, the focal position is D=0.998 mm, acorrection wavefront becomes a correction pattern as shown in FIG. 21,and the phase modulation amount of the correction wavefront is reducedto approximately 14 radians.

Results of measurement of a focused state in the processing target 60using this correction pattern are shown in FIG. 22. FIG. 22 show resultsof observation of a focusing portion when a laser beam with a wavelengthof 660 nm is focused on the inside of acryl. FIG. 22(a) shows a resultof measurement of a focused state before correction, and FIG. 22(b)shows a result of measurement of a focused state after correction of thesecond embodiment. As shown in FIG. 21, due to the small phasemodulation amount, aberration is sufficiently corrected as shown in FIG.22(b).

FIG. 23 and FIG. 25 show results of observation of a cut surface of aprocessing target 60 cut after laser processing. In the laser processingin FIG. 23 and FIG. 25, by irradiating a laser beam from the direction Zand scanning the laser beam in the direction Y on the processing target60, three modified layers 60 a, 60 b, and 60 c were formed. FIG. 23shows a cut surface in the case where the aberration-correcting methodof the second embodiment of the present invention was not used in laserprocessing, that is, a cut surface after laser processing using a laserbeam whose aberration has not been sufficiently corrected as shown inFIG. 22(a). On the other hand, FIG. 25 shows a cut surface in the casewhere the aberration-correcting method of the second embodiment of thepresent invention was used in laser processing, that is, a cut surfaceafter laser processing using a laser beam whose aberration wassufficiently corrected as shown in FIG. 22(b). Referring to FIG. 23, asdescribed above, aberration correction is insufficient in laserprocessing, so that the modified layers 60 a, 60 b, and 60 c are notuniform, and laser processing is insufficient. On the other hand,referring to FIG. 25, aberration correction was sufficiently performedin laser processing, so that the modified layers 60 a, 60 b, and 60 care uniform, and laser processing is sufficiently performed. Aprocessing mark of FIG. 25 which is a result of the second embodiment issimilar to a processing mark of FIG. 24 which is a result of the firstembodiment, so that it can be confirmed that both of these embodimentshave effects equivalent to each other.

Further, according to the aberration-correcting method of the secondembodiment and the aberration-correcting device and theaberration-correcting program of the present embodiment, correctionwavefronts for a plurality of processing positions are obtained inadvance, and high-order polynomial approximation of these correctionwavefronts is performed, so that a proper correction wavefront can beobtained by only performing an arithmetic operation according to theapproximate expression. As a result, the time for re-calculating thecorrection wavefront when changing the processing depth can beshortened, and a lowering in the processing rate can be reduced.Further, even for an arbitrary processing position different from theprocessing positions obtained in actuality by searching, a propercorrection wavefront can be obtained.

The present invention is not limited to the above-described embodiments,but can be variously varied. For example, in the embodiments, aphase-modulation type reflective SLM 40 which applies a voltage to anindependent pixel is used, however, even when a phase-modulation typetransmissive SLM which applies a voltage to an independent pixel isused, the same advantages can be obtained. Even when a deformable mirroris used, the same advantages can be obtained.

Further, it is also possible that between the SLM 40 and the objectivelens 50, a relay lens system consisting of one or two or more lenses isdisposed so that the modulation surface of the SLM 40 and the entrancepupil plane of the objective lens 50 are substantially in an imageforming relationship. Accordingly, the wavefront modulated by the SLM 40is propagated to the objective lens 50 without Fresnel diffraction, sothat excellent aberration correction can be performed. When themodulation surface of the SLM 40 is larger than the pupil plane of theobjective lens 50, if the image forming system commonly serves as areduction system, the light amount of the laser beam can be effectivelyused, and the effective region of the SLM 40 can be sufficiently used.

In the present embodiment, single-point processing is shown by way ofexample, however, the concept of the present invention is alsoapplicable to multi-point processing for processing a plurality ofprocessing points which are three-dimensionally distributed. Forexample, in two-point processing, to the phases of two Fresnel lenspatterns with different focal positions, phases of correction wavefrontsset by considering the processing depths are added. When the obtainedphases of the patterns of the two points are defined as Φ_(A) and Φ_(B),by extracting only the phases from exp(Φ_(A))+exp(Φ_(B)), a hologrampattern of the spatial light modulator is obtained. Thus, the spatiallight modulator has advantage in three-dimensional processing.Specifically, a large number of focal points can also be generated atdifferent positions in a plane in the depth direction by modulatingincident light, and in comparison with processing in which single-pointprocessing is repeated, the throughput of processing can be improved.

In this multi-point processing, as in the case of the presentembodiment, for focusing on different processing depths, correctionwavefronts corresponding to the processing positions are also obtained.In this case, by performing searching using the expressions (7) and (4)to (6), etc., correction wavefronts can be obtained so that the focalpoints of the laser beam are positioned within a range of longitudinalaberration occurring inside the medium and the phase modulation amountsof the correction wavefronts becomes minimum, however, this searchingtakes a great deal of time. Therefore, by applying theaberration-correcting method using the above-describedaberration-correcting device 90 to this multi-point processing, the timeto be taken for re-calculating the correction wavefront when changingthe processing depth can be shortened, and a lowering in the processingrate can be reduced.

In this multi-point processing, Fresnel zone plate patterns (consistingof two values of 0 and π) may also be used instead of theabove-described Fresnel lens patterns. When adding phases of correctionwavefronts set by considering the processing depths of the Fresnel lenspatterns to the phases of the Fresnel lens patterns, for example, aphase of a grating pattern or an arbitrary CGH pattern for generatingmultiple points within a plane with a uniform depth may be added to theFresnel lens patterns.

In the present embodiment, an aberration-correcting method in a laserprocessing device is described, and this aberration-correcting method isapplicable to various optical system devices. For example, theaberration-correcting method and laser irradiation method of the presentinvention are also applicable to laser irradiation devices such asmicroscopes. Among various microscopes, the present invention isparticularly suitable for a laser-scanning microscope. Hereinafter, anexample of a laser-scanning microscope will be described as a laserirradiation device and a laser irradiation method of the presentinvention.

For example, a laser-scanning microscope scans a focal position of alaser beam not only in a direction perpendicular to the optical axisdirection but also in the optical axis direction. Specifically, thelaser-scanning microscope generates a focal point not only on thesurface but also the inside of a measuring target. In this case, thefocal point expands and the peak intensity decreases due to aberration,and the resolution and the image contrast lower. By applying theaberration-correcting method and laser irradiation method of the presentembodiment to this laser-scanning microscope, even if an SLM the phasemodulation amount of which is limited is used, the concentration of afocused laser beam on the inside of the measuring target can be improvedand an image with high resolution and high image contrast can bemeasured even at a deep position. In a confocal microscope or amulti-photon laser-scanning microscope as a kind of laser-scanningmicroscope, if the peak intensity at the focal position of irradiationlight decreases, the measured light intensity dramatically decreases, sothat the aberration correction is especially effective. Like thelaser-scanning microscope, in some imaging devices which obtain an imageby scanning a focusing beam, a light source such as an SLD(Super-Luminescent Diode) that has a narrow wavelength band and highspatial coherence like laser is used, and to such a light source, theaberration-correcting method and laser irradiation method described herecan be applied.

The aberration-correcting method of the present invention is applicableto various microscopes as well as the above-described laser-scanningmicroscope, and is preferably applicable to an imaging device such as amicroscope which widely illuminates a measuring target and detects themeasuring target by an image sensor. In this kind of microscope, thelight source is not limited to a laser beam, and noncoherent light maybe used. Thus, the aberration-correcting method of the present inventionis also applicable to a microscope (light irradiation device) usingnoncoherent light. Hereinafter, as a light irradiation device relatingto the present invention, an example of this kind of microscope will beshown.

FIG. 26 is a light irradiation device according to an embodiment of thepresent invention, showing an example of a microscope which widelyilluminates a measuring target and images the measuring target by animage sensor. The microscope 1B shown in FIG. 26 includes a light source10B, a condenser lens 20B, a mirror 22B, an objective lens 50, relaylenses 24B and 26B, a prism mirror 30, a spatial light modulator 40, animaging lens 28B, and a camera (image sensor) 70B. In the lightirradiation device of this embodiment, as a specimen (medium) 60B of ameasuring target, living tissue in a container with water is assumed.

The light source 10B is, for example, an illumination such as anincandescent lamp. Light from the light source 10B is converted intoparallel light by the condenser lens 20B, and reflected by the mirror22B to widely illuminate the specimen 60B. Transmitted andforward-scattered light from the specimen 60B is made incident on theobjective lens 50, and light outgoing from the objective lens 50 isguided onto the SLM 40 via the relay lenses 24B and 46B and the prismmirror 30. The light reflected by the SLM 40 is guided to the camera 70Bvia the prism mirror 30 and the imaging lens 28B, and forms an image ofthe specimen 60B on the surface of the camera 70B. In this embodiment,the relay lenses 24B and 26B are provided between the objective lens 50and the SLM 40 so that the entrance pupil plane of the objective lens 50and the SLM 40 have a conjugate relationship. The lenses 50, 24B, 26B,and 28B are disposed so that the specimen 60B and the surface of thecamera 70 have an image forming relationship.

In the light irradiation device of this embodiment, the specimen 60B isregarded as a group of points, and each point is regarded as a secondarylight source. Specifically, the specimen 60B is regarded as a group ofsecondary point light sources. Thus, between the specimen 60B that is agroup of secondary point light sources and the camera 70B, theaberration-correcting method of the present invention is applied.

From each secondary point light source, spherical wave light is emitted,and is converted into substantially planar wave light by the objectivelens 50. Here, the specimen 60 is in water, so that light emitted fromeach secondary point light source has spherical aberration due torefractive index mismatch between air and water, and light to outgo fromthe objective lens 50 becomes a planar wave that includes wavefrontaberration expressed by the expression (7). Therefore, when forming animage on the camera 70B by the imaging lens 28B, the image is influencedby the aberration, and the spatial resolution and contrast of the imageare lowered.

Light that outgoes from the objective lens 50, and the planar wave lightincluding wavefront aberration is transmitted onto the SLM 40 by therelay lenses 24B and 26B. At this time, by applying phase modulationexpressed by the expression (7) to the SLM 40, the aberration can beremoved. Accordingly, light to outgo from the SLM 40 becomes light of anearly perfect planar wave from which wavefront aberration has beenremoved. As a result, when forming an image on the camera 70B by theimaging lens 28B, an image without aberration is formed, and theresolution and contrast of the image can be improved.

In the laser processing devices of the first and second embodiments, alight source is above the prism mirror 30, and light is propagated fromthe upper side to the lower side, however, in the light irradiationdevice of the present embodiment, the light propagation direction isreverse thereto. A light source is below the objective lens 50, andlight is propagated from the lower side to the upper side. In the scopeof classical optics, light propagation is invariable with respect totime reversal, so that it is evident that aberration correction isperformed regardless of the light propagation direction as long as theconfiguration is the same. However, in the laser processing devices ofthe first and second embodiments, the focal point is on the opticalaxis, however, in the light irradiation device of the presentembodiment, the light source 10B is not only on the optical axis butalso on other positions, so that an influence of this will beconsidered.

Two luminous points in the specimen 60B, that is, a luminous point A onthe optical axis and a luminous point B out of the optical axis areconsidered. Light to be emitted from the luminous point A is influencedonly by spherical aberration because the luminous point A is on theoptical axis, and the aberration is satisfactorily removed by theaberration correction of the present invention. On the other hand, lightto be emitted from the luminous point B includes not only sphericalaberration but also other aberrations because the luminous point B isout of the optical axis. However, in a normal microscope, theobservation field is narrow. As long as the luminous point B is includedin the observation field, the deviation from the optical axis is small,and aberrations other than spherical aberration are sufficiently small.Therefore, aberration is also satisfactorily removed from the light tobe emitted from the luminous point B by the aberration correction of thepresent invention.

In the light irradiation device of this embodiment, transillumination isshown, however, the aberration-correcting method of the presentinvention is also applicable to epi-illumination. Further, in the lightirradiation device of this embodiment, the relay lenses 24B and 26B areused, however, these can be omitted. In this case, it is preferable thatthe SLM 40 and the objective lens 50 are made close to each other. Inthe light irradiation device of this embodiment, as the light source10B, an incandescent lamp is used, however, as the light source 10B,other white light sources, lasers, SLDs, and LEDs, etc., can also beused. Further, it is also possible that light the wavelength band ofwhich is limited by using a band-pass filter in the white light sourceis used as illumination light.

The present embodiment is described by using an example of a processingtarget with a spatially uniform refractive index, however, the presentinvention is also applicable to a case where the refractive index can beregarded as substantially uniform. For example, in the case of atransparent substrate having a thin film such as an anti-reflection filmformed on the surface and the back surface, the thin film layer is thin,and aberration caused by the thin film is small and negligible.Alternatively, when a processing target formed by laminating and bondingidentical thin glasses is used, a glass layer and a bonding layer arealternately laminated in the depth direction, and the thickness of thebonding layer is thin and the refractive index difference from glass issmall, and aberration occurring in the bonding layer is small andnegligible. Further, the above-described embodiments are described byusing a case where light is focused on the inside of a processingtarget, however, the present invention is also applicable to focusingnear the back surface. For example, in the manufacturing process of aliquid crystal panel, there is a case where a wiring pattern formed onthe back surface of a glass substrate is cut by a laser beam madeincident from the surface, and the present invention is also applicableto this case.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a purpose involving a laserirradiation position deep in a medium and requiring a high concentrationof a focused of a laser beam.

REFERENCE SIGNS LIST

-   1, 1A: Laser processing device (laser irradiation device, laser    focusing device)-   1B: Light irradiation device (microscope)-   10, 10B: Light source-   20, 20B, 24B, 26B, 28B: Lens-   30, 22B: Mirror-   40: Spatial light modulator (SLM)-   50: Objective lens (focusing means, condenser lens)-   60: Processing target (medium)-   70: Measurement system-   70B: Camera (image sensor)-   80: Control unit-   90, 90A, 90B: Aberration-correcting device-   91: First correction wavefront generation unit (first correction    wavefront generation means)-   92: First polynomial approximation unit (first polynomial    approximation means)-   93: Second polynomial approximation unit (second polynomial    approximation means)-   94: Third polynomial approximation unit (third polynomial    approximation means-   95: Storing unit (storing means)-   96: Second correction wavefront generation unit (second correction    wavefront generation means)-   100: Recording medium-   200: Computer-   202: Reading device-   206: Memory-   208: Display device-   210: Mouse-   212: Keyboard-   214: Communication device-   220: Computer data signal-   P100: Aberration-correcting program-   P10: First correction wavefront generation module-   P20: First polynomial approximation module-   P30: Second polynomial approximation module-   P40: Third polynomial approximation module-   P50: Storing module-   P60: Second correction wavefront generation module

The invention claimed is:
 1. A laser processing method for processing aprocessing target using a laser processing device, which focuses a laserbeam on the inside of a transparent processing target, including afocusing means for focusing a laser beam on the inside of the processingtarget and a spatial light modulator for correcting aberration of laserbeam, comprising: setting a focal position inside the processing target;setting a relative movement distance of the processing target so that afocal point of the laser beam is positioned in an aberration rangegenerated inside the processing target; calculating a correctionwavefront so that the laser beam is focused on the focal position, anddisplaying the correction wavefront on the spatial light modulator;relatively moving the focal position so that the distance between theprocessing target and the focusing means becomes the relative movementdistance; irradiating the laser beam onto the focal position in theprocessing target, thereby forming a modified layer in the processingtarget; and cutting the processing target, thereby positioning themodified layer at a cut surface of the processing target.
 2. The laserprocessing method according to claim 1, wherein the setting the relativemovement distance of the processing target includes setting the focalpoint of the laser beam so that the focal point of the laser beam ispositioned within a range more than n×d and less than n×d+Δs from theincidence plane of the processing target, when the refractive index ofthe processing target is defined as n, a depth from an incidence planeof the processing target to the focus of the focusing means on theassumption that the refractive index n of the processing target is equalto that of a focusing means atmospheric medium is defined as d, and amaximum value of longitudinal aberration caused by the processing targetis defined as Δs.
 3. A laser processing method for processing aprocessing target using a laser processing device including a lightsource for generating a laser beam, a spatial light modulator formodulating the phase of a laser beam from the light source, and acondenser lens for focusing the laser beam from the spatial lightmodulator on a predetermined focal position inside the processingtarget, comprising: setting the focal position inside the processingtarget; setting a relative movement distance of the processing target sothat the focal position is positioned within a range in whichlongitudinal aberration is present inside the processing target when theaberration is not corrected; calculating a correction wavefront so thatthe laser beam is focused on the focal position, and displaying thecorrection wavefront on the spatial light modulator; relatively movingthe focal position so that the distance between the processing targetand the condenser lens becomes the relative movement distance; andirradiating the laser beam from the light source onto the focal positionin the processing target, thereby forming a modified layer in theprocessing target; and cutting the processing target, therebypositioning the modified layer at a cut surface of the processingtarget.